View Full Version : Pressure testing with nitrogen

22-07-2015, 12:13 PM
A colleague mentioned the other day how he tests with nitrogen.
Usually pressure can drop or rise with different ambient temps.

This may not be a new idea but isolating pressure gauge from system while system under pressure test, then say next day pressure is checked & compared, if gauge pressure is lower than day before, open valve & check system pressure compared to gauge pressure.
If the same system & isolated gauge, no leaks.

sound feasible?

22-07-2015, 04:21 PM
Nitrogen stays at the same pressure with normal ambient temp.
It has big molecules so small leaks are a problem to find, adding a small amount of helium can help with the tiny leaks.

22-07-2015, 05:10 PM
I usually put a calibrated pressure gauge on the system and isolate it from the system once I have the desired pressure.

when I check the next day it's easier to see if the needle moves when you open the isolating valve.

22-07-2015, 06:18 PM
Hi Ranger.
I have witnessed as much as 5 psi differential over a 24hrs on Type 22 destroyers when I worked on them.
Especially when there is 20,000 tons of steel hull and infrastructure ect.
Which will be cool in the evening and heats up during the course of the day.
(Especially when in dry dock!)

So yes, Technically Nitrogen system pressures can fluctuate a little. but in most circumstances you are not going to see a rise only a drop in pressure.
Isolating the gauge lines is always a good option with maybe a dedicated Gauge on as short a line as possible is also good practice.

Basically what Tyco says is spot on.
Chemi is also correct although he forgot to mention that you need a special leak detector for detecting helium though, I believe?
You can buy nitrogen with a trace of Helium nowadays (in the U.K. at least!)


22-07-2015, 09:30 PM
I have been spending some time trying to find a reliable source or a diagram that would show the thermal dynamics of OFN in a gaseous state... I learned a few things I didn't know, but I didn't find what I was looking for :)

there was a movie quite a few years back, might have been fast&furious, but anyway, it created a hype that having OFN in your tires would make them last longer *sigh* we all know that all gasses expand or contract depending on temperature, some more or less than others.
The air we breathe every day is in the range of 75-80% N2 i think (No more googling now :))

What I did find was that OFN would have a 0.068 Kg/cm2 change in pressure for every 6K change in temperature. <- don't quote me on this :)

So it's really one big unknown, because depending on the size of the system, changes in pressure could vary.
On a small AC unit, the temperature change could be significant, while the pressure would hardly have changed at all.
same thing on a larger industrial system, the temperature drop could be insignificant, but because of the larger volume and exposed surfaces, the pressure drop might be significant.

If I pressure test a system overnight and notice a slight drop in pressure and I can't find anything with soapy water, I'll vacuum the system and I'll just make the drop test longer to make sure :)

But then again I've had systems hold a drop test for 24 hours at 3mbar and still start leaking somewhere when I put ammonia in :)

22-07-2015, 10:10 PM
Thanks for replies.
Mir came up while pressure testing some pipework on new ammonia chiller extension.
I gues volume would take awhile to react to temperature change.
It was mainly done as strength test as all connections fully welded using steel pipe of course,valves Danfoss ICF, ICS etc.